As a factor of fuel economy in tires, rolling resistance of tires and air resistance is known. The major cause of rolling resistance of tires is energy loss due to repeated deformation of rubber during traveling. In order to reduce the rolling resistance, it is proposed to use a tread rubber having low energy loss (low tan δ).
When the tread rubber having low energy loss is used, although the rolling resistance is reduced, grip performance (especially, wet grip performance) is, however, lowered, and there is another problem that the abrasion resistance is deteriorated. As shown in Patent Documents 1 and 2 below, studies of the tread rubber composition having a reduced rolling resistance while improving wear resistance has been developed. Unfortunately, only improvement depending on rubber composition gives insufficient effect. Thus, another approach to improve rolling resistance in terms of other than improvement of rubber composition is strongly demanded.
In view of these circumstances, the present inventors who had conducted studies were able to find the following. When a tire section width is reduced while maintaining its tire outer diameter, a tread width is also decreased accordingly. Accordingly, the volume of the tread rubber is reduced. As a result, the energy loss caused by the tread rubber is reduced, and the weight of the tire is also reduced. Furthermore, when viewing a vehicle from its front, a tire exposed area appearing downwardly from a bumper edge is reduced according to the reduced tire section width. This makes it possible to reduce the air resistance of the tire. Furthermore, when a bead diameter is enlarged while maintaining its tire outer diameter, a sidewall region which tends to deform largely during traveling narrows. As a result, a reduction in energy loss in the sidewall region as well as the weight of the tire can be achieved.
Therefore, it has been confirmed that a narrow width and large bead diameter tire in which the tire section width is reduced while enlarging the bead diameter has significantly improved fuel economy through reduction of energy loss in the tread portion and the sidewall portion, reduction of tire mass, and reduction of air resistance.
Unfortunately, as a result of further study of the inventors of the present invention, the above-mentioned narrow width and large bead diameter tire has also reduced tread width according to the reduced tire section width. With this, a shoulder portion of the tire is subjected to high ground contact pressure and a long ground contact length during load application, and thereby there was a new problem to be solved that shoulder wear had generates at the time of cornering.